Exotic predators swallow the Southwest's native frogs

  • A bullfrog devours a smaller bullfrog

    Julia Rosen photo
  • Cecil Schwalbe

    Phil Rosen photo
  • Phil Rosen with a coach-whip snake

    Shawn Sartorius photo
  LEWIS SPRINGS, Ariz. - Phil Rosen is knee-deep in a disaster this spring day. Just a few years ago, native leopard frogs filled algae-covered pools in this side drainage of the San Pedro River, one of the last free-flowing rivers in the Southwest, 70 miles southeast of Tucson.


Now, Rosen keeps turning up bad news. He wades up the trickle, plunging his collection net under the watercress. "Crayfish," the biologist says, showing the mucky contents of his net. "Two more crayfish," he says, wishing he'd find a native frog.


Rosen is disappointed: Another aquatic system looks to have been usurped by the West's menagerie of non-native invaders.


For Rosen, a doctoral candidate in herpetology (the study of reptiles and amphibians) at the University of Arizona, the story is getting old. Over the past 20 years, many of the populations of native amphibians he studies in Arizona and New Mexico have disappeared. Four of Arizona's five remaining species of native aquatic frogs have hit critical status in recent years. And the biggest factor, contend Rosen and his colleague Cecil Schwalbe, a University of Arizona ecologist, is the rapid spread of non-native bullfrogs, crayfish and predatory fishes.


Other factors also contribute to killing native frogs and salamanders, including pesticide runoff, smelter-related acid rain and increased ultraviolet radiation caused by our depletion of the ozone layer. But in the case of Arizona's amphibians, Rosen and Schwalbe say radical, man-made habitat changes have provided a perfect environment for native-gobbling aliens.





A regional mix-up


Left to themselves, the natural wetlands and rivers might have resisted exotic invasions. But they were not left alone.


Water developers - local, state and federal - built hundreds of dams, reducing and flattening out the radical swings in water flow.


Ranchers dug thousands of stock-watering tanks near perennial springs, creating thousands of ponds where none existed before. Flows to marshes were diverted or impounded, or entrenched in channels.


And the Arizona Game and Fish Department, like other state game agencies, promoted sport fishing by stocking new artificial lakes with non-native bass, trout and even, until the early 1970s, bullfrogs, a native of the eastern United States.


Throw in a human population boom and "you have a mess," says Schwalbe.


"Before all this habitat modification, there wasn't much standing water in Arizona," he says. "But now you've created a bunch of perfect environments for all the Great Plains and Southeastern predators we've brought in. No wonder we've got an explosion of exotic invaders."


Or, as Rosen says, "Abrupt flooding and frequent drying define native communities in the Southwest. Take that away and you take away the natives' advantages."


The changes have already decimated the region's native fish populations. Forty kinds of non-native fish have overwhelmed 33 natives, wiping out five of them and leaving 19 others threatened or endangered. Now exotic fishes, voracious frogs and crayfish are wiping out amphibians.


The invaders have moved beyond the state's lowland reservoirs and rivers to take over adjacent wetlands and even infiltrate the higher waters of southeastern Arizona's "sky island" mountain ranges.


Wherever they go, they chew through local populations of smaller leopard frogs, Mexican garter snakes and Sonoran turtles. In the larger streams and impoundments, the fish scarf up larval leopard frogs. Elsewhere, crayfish eat native frogs and their tadpoles as well as other organisms, including aquatic vegetation. And the bullfrogs ... well, don't get Cecil Schwalbe started.


"Bullfrogs are the primary threat to the remaining native frogs in Arizona," says Schwalbe. "I mean, they say bullfrogs are competitors. Hell, they just eat the leopard frogs. We've opened their stomachs and found virtually everything they exist near: bats, snakes, turtles, even a red-wing blackbird. We've found them sitting every meter along the pond edge, by the hundreds. They're monsters!'


After 15 years of study, Schwalbe and Rosen say the spread of bullfrogs has now relegated native leopard frogs mainly to scattered and peripheral pockets - two or three canyons here, a number of remote cattle tanks there. So precarious looks the future of one species, the Chiricahua leopard frog (Rana chiricahuensis), that U.S. Fish and Wildlife Service officials say they will list it as threatened or endangered later this year.





A trickle of hope


And yet Rosen and Schwalbe harbor a hope: Though the spread of invasive species on dry land may be irreversible, in the aquatic system victories remain possible.


The two first experimented with eliminating bullfrogs from a test wetland at the San Bernardino National Wildlife Refuge in the extreme southeast corner of Arizona. They trapped, speared and shot hundreds, but they couldn't match the evasiveness - or fecundity - of the bullfrogs, which rebounded quickly. They also moved leopard frogs into fenced, newly created, bullfrog-free ponds. Such projects on a remote federal wildlife refuge have allowed the researchers to develop recovery techniques without inconveniencing or alienating ranchers already smarting from federal land grazing shut-downs caused by environmental litigation.


But Rosen and Schwalbe are also working with ranchers. In one of these projects, the researchers have teamed with ranchers Matt and Anna Magoffin, Arizona Game and Fish frog specialist Mike Sredl and the Malpai Borderlands Group, a private nonprofit confederation of ranchers in southeast Arizona, to protect a beleaguered population of Chiricahua leopard frogs on a private ranch adjacent to the San Bernardino refuge.


For months during the drought year of 1994-95, the Magoffins and their children hauled a thousand gallons of water a week to keep the frogs wet and alive in an earthen cattle pond. Since then the researchers and their collaborators have received a grant to secure more permanent water sources and drill additional wells. The upshot: the Magoffins have two new wells on their place, and the frogs have a new lease on life in at least one wet spot.


"We realized that 80 to 90 percent of the surviving leopard frogs are in stock tanks, so the idea is to use the tanks as permanent stop-gaps," says Schwalbe. "At least then we'll maintain some frogs and have them for translocation to other likely areas."


But these projects are "triage," as Rosen puts it - emergency interventions to save teetering populations. He and Schwalbe hope to score a more lasting counterstrike against non-natives by restoring the cycles of desert wetlands, which brim with floodwaters during rainy seasons but dry out at other times. This, the researchers believe, would let natural conditions select for native fauna.


"Leopard frogs are very much able to persist with natural drying, while bullfrog adults have a harder time," Rosen explains. In addition, he says, flash floods that native frogs can hop away from may sweep bullfrog tadpoles and maladapted slow-water fish away.


Rosen and Schwalbe advocate removing artificial water developments in wetlands wherever possible. In some places this would involve "active management." At the San Bernardino refuge, for instance, refuge managers try to halt erosion by encouraging arroyos like Black Draw to fill with sediment. As the eroded channel fills, natural cienega and stream conditions will return, according to the researchers.


In other cases, restoration can be accomplished by doing nothing. Rosen has urged the U.S. Forest Service simply to forego dredging the silt out of Rucker Lake, a large artificial pond in the Chiricahua Mountains that filled with ash and debris in the wake of the 1994 Rattlesnake Fire. That non-action would not only save tens of thousands of dollars, but would also permit Rucker Creek to exist without the lake habitat that previously supported introduced trout.


"They should just leave it. Threatened fishes are already coming back in the canyon," says Rosen. So far, he has gotten his wish, but local citizens in nearby Douglas continue to lobby the Forest Service to reopen what was a popular sports fishery in the lake.


"Fishing at that lake was tradition and even economic development in Douglas," says Art Macias, the town's economic development director. "When they say putting the lake back might disturb other species in the streams, I think it's just another delaying tactic. That wasn't a problem before. I don't see why it is now."


Getting the public behind wetlands restoration won't be easy. Still, the herpetologists hope to organize a multi-agency wetlands restoration task force. But the initiative, Rosen adds, will never work without enlisting private landowners.


"We need them to help us help these frogs," Rosen says, "since all those stock ponds and swimming holes are both the boon and the bane of the situation now." n





Mark Muro lives in Tucson, Arizona. He writes editorials for The Arizona Daily Star.





You can contact ...


*Phil Rosen, University of Arizona, Tucson, AZ 85721 (520/621-3187); rosen@ccit.arizona.edu.